treat void as a top type, analyzer fix for #28763

pkg/dev_compiler/test/not_yet_strong_tests.dart

 import 'dart:collection';
 
 /// The set of tests that are not yet strong mode safe.
 final notYetStrongTests = new HashSet<String>.from([
   'language/abstract_exact_selector_test_01_multi',
   'language/abstract_factory_constructor_test_00_multi',
   'language/abstract_getter_test_01_multi',
   'language/abstract_runtime_error_test_01_multi',
   'language/abstract_runtime_error_test_02_multi',
   'language/abstract_runtime_error_test_03_multi',
@@ skipping 86 matching lines @@
   'language/async_await_syntax_test_d10a_multi',
   'language/async_await_syntax_test_e1_multi',
   'language/async_await_syntax_test_e2_multi',
   'language/async_await_syntax_test_e3_multi',
   'language/async_await_syntax_test_e4_multi',
   'language/async_await_syntax_test_e5_multi',
   'language/async_await_syntax_test_e6_multi',
   'language/async_await_syntax_test_e7_multi',
   'language/async_await_syntax_test_e8_multi',
   'language/async_await_syntax_test_e9_multi',
-  'language/async_or_generator_return_type_stacktrace_test_01_multi',
-  'language/async_or_generator_return_type_stacktrace_test_02_multi',
-  'language/async_or_generator_return_type_stacktrace_test_03_multi',
   'language/async_return_types_test_tooManyTypeParameters_multi',
   'language/async_return_types_test_wrongReturnType_multi',
   'language/async_return_types_test_wrongTypeParameter_multi',
   'language/await_backwards_compatibility_test_await1_multi',
   'language/await_backwards_compatibility_test_await2_multi',
   'language/await_backwards_compatibility_test_await3_multi',
   'language/await_backwards_compatibility_test_await4_multi',
   'language/await_test',
   'language/bad_constructor_test_00_multi',
   'language/bad_constructor_test_01_multi',
@@ skipping 1141 matching lines @@
   'language/naming3_test',
   'language/naming_test',
   'language/new_expression_type_args_test_00_multi',
   'language/new_expression_type_args_test_01_multi',
   'language/new_expression_type_args_test_02_multi',
   'language/new_prefix_test_01_multi',
   'language/no_such_constructor2_test',
   'language/no_such_constructor_test_01_multi',
   'language/no_such_method2_test',
   'language/no_such_method_dispatcher_test',
-  'language/no_such_method_mock_test',
   'language/no_such_method_test',
   'language/non_parameterized_factory2_test',
   'language/non_parameterized_factory_test',
   'language/not_enough_positional_arguments_test_00_multi',
   'language/not_enough_positional_arguments_test_01_multi',
   'language/not_enough_positional_arguments_test_02_multi',
   'language/not_enough_positional_arguments_test_03_multi',
   'language/not_enough_positional_arguments_test_05_multi',
   'language/not_enough_positional_arguments_test_06_multi',
   'language/not_enough_positional_arguments_test_07_multi',
@@ skipping 1132 matching lines @@
   'lib/html/mutationobserver_test',
   'lib/html/postmessage_structured_test',
   'lib/html/resource_http_test',
   'lib/html/transferables_test',
   'lib/html/webgl_1_test',
   'lib/html/wrapping_collections_test',
   // TODO(jmesserly): these are both under "dart:html" as well.
   'js_test',
   'js_util_test'
 ]);